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266                       HORNS OF TITA&OTHERTUM                       CHAP.
together with the long and divergent horn cores, must have given
to the living animal a most bizarre appearance. It is an interest-
ing fact that this animal, though a Perissodactyle, agrees with the
Artiodactyla in the nineteen dorso-lumbar vertebrae, of which seven-
teen bear ribs.
The genus further agrees with the Artiodactyles, in the
structure of the carpus. The toes of the fore-limb are four,
those of the hind-limb three; but while the hind-limb is un-
doubtedly Perissodactyle in the arrangement of its component
parts, the fore-limb shows a hint of an Artiodactyle mode of
structure. This limb is paraxonic, the axis of the limb passing
between, the two middle digits. It may be that this genus
represents more nearly than any other Perissodactyle or Artio-
dactyle the primitive stem from which both have diverged, though,
of course, it is not old enough to be very near to the actual
ancestor. The molar dentition is the typical one; the incisors
seem to vary as to their presence or absence, and, if present, in
their numbers. In comparing the older with the more recent
forms it is noteworthy that there has been an increase of size
exactly as there has been during the evolution of the Camels and
some other groups of Ungulates. As steady mentioned, the size
of the horn cores also increases until it culminates in the extra-
ordinary species, T. platyceras and T. ramosum, in which these are
tialf as long as the skull, flattened in form, and connected at
their bases by a "web" of bone. Arrived at this amount of
specialisation the genus Titanotherium apparently exhausted its
capacities for modification and ceased to be. The many
generic names may be explained by sexual differences on the one
band and an incomplete knowledge of connecting links on the
PaLaeosyops is somewhat like a Tapir in build, the skull
especially resembling that of the Tapir. As in Titanotherium
the molar teeth, instead of having an outer wall formed by fused
cusps, iiave a W-shaped outer wall on one side and two or one
cusps on the opposite side. It is, moreover, an Eocene form, and
ia corresponcbnee with, its greater age is more primitive in some
of stracfctrre, fox example, in the absence of horns and in
full Cental formula. The fore-limbs are four-toed, the hind